How to get client script to run before rendering on Network Render
I expect it is the way the addon runs. The python script is in control as it reads the file and starts rendering, only after it has finished does blender sit idle and go back to running registered text blocks from the open file step.

First I would suggest you setup the slaves to always start with gpu rendering enabled.

Failing that, you could adjust the netrender addon on each slave. You will find in netrender/operator.py that rendering starts either on line 80 or 97 you could specifically run your script before the render step or add your gpu enable code to the addon.

text = bpy.data.texts['myStartscript.py']
if text is not None:
    exec(compile(text.as_string(), 'errors.log', 'exec'))
 bpy.ops.render.render('INVOKE_AREA', animation=True)